When designing integrated circuits, it is often necessary to choose from among several circuit alternatives, frequently each circuit alternative having quite different characteristics.
Choosing one circuit solution over the others possible involves the exclusion of the other circuit alternatives. However, it may happen that the solution chosen during the design phase is not practically the best one. Additionally, after one circuit solution has been chosen, switching to another alternative solution initially discarded involves remaking all or at least some of the photolithographic masks used in the manufacturing process. This obviously causes an increase in the manufacturing costs. Moreover, whenever one or more photolithographic masks have to be redesigned, there is always the possibility of incurring new errors. This causes an increase in the testing phase.
Therefore, in practice the designer of an integrated circuit provides, whenever it is possible, for the implementation of more than one circuit alternative for at least some of the circuit blocks. The provision of such circuit alternatives, or "options," involves providing alternative metal interconnection lines that can be disabled during the testing of the device. For example, assuming that it is desired to provide more than one possibility for the control signal of a generic circuit block, it is possible to provide a multiplexer supplied with several signals, among which the one most suitable for driving the circuit block is to be determined. During the testing of the device, it is possible to deliberately interrupt some of the electrical interconnection lines in such a way that the multiplexer output switches among all the possible driving signals. Once the best circuit alternative has been determined, the photolithographic mask defining the metal interconnections, which is typically one of the last masks in the manufacturing process flow, will be redesigned so as to render the chosen alternative definitive.
However, configuring the multiplexer, or more generally configuring the combinatorial circuits that allow experimentation of more than one option, by interrupting the metal interconnection lines has the disadvantage of leaving the inputs of the logic circuits connected to the interrupted interconnection lines floating, and not at a well-defined logic level. This causes indetermination of the integrated circuit performance and increases the device's power consumption because of the absence of well-defined logic levels.
In order to configure the combinatorial circuits that choose from among the several circuit alternatives possible, it is also possible to use programmable nonvolatile memory elements. However, this approach normally complicates the manufacturing process because the process technology for the integration of the memory elements must be used.
Similarly, other testing solutions may be implemented. For example, using so-called "third-levels" of voltage applied to the input terminals of the integrated circuit. However, these solutions require the use of specially-designed circuits, which result in an increase in the design costs and in the device area.